1. Field of the Invention
The present invention relates generally to gyroscopes and more particularly to gyroscopes which employ optical waveguides.
2. Description of the Prior Art
Optical rotation sensors, also known as gyroscopes, are based on the well-known nonreciprocal optical effect known as the Sagnac effect. Simply stated, if two beams of light travel in opposite directions in a well-defined, stationary closed loop, the optical pathlengths associated with the clockwise (CW) and counterclockwise (CCW) beams will be the same. However, if the system is rotating, one beam of light will be traveling in the direction of rotation, and the other beam of light will be traveling opposite to the direction of rotation. This breaks the symmetry of the system, that is, it introduces nonreciprocity into the system. In this case, the optical pathlengths of the clockwise and counterclockwise beams become different, with the difference proportional to the rotation rate, as well as the area enclosed by the loop with its normal parallel to the axis of rotation.
There are several types of optical gyroscopes which are based on the Sagnac effect.
In Ring Laser Gyros (RLGs), the CW and the CCW beams are passed in opposite directions around a resonator loop and each direction is kept at resonance by modifying the effective optical length around the loop for example by directly changing the length of the resonator loop, by a piezo-electric transducer. The rate of rotation applied to the gyro may be determined by detecting the frequency difference between the resonant CW and CCW beams and dividing the difference by a scale factor.
A conventional RLG consists of a dimensionally-stable polygon with at least three edges with mirrors positioned to form a closed optical path. This optical path is established within a vessel, in which a gas discharge path is provided. The gas discharge path in the ring resonator acts as an optical amplifier.
With sufficient gain of the gas discharge, the system will operate as a ring laser. Counterpropagating laser beams inside the cavity create a standing wave pattern at the resonance As frequency of the cavity. A fraction of the generated laser beams are coupled out by means of a partially transparent mirror, and form an interference pattern which is detected by two side-by-side detectors.
As the gyro is rotated, the optical path length changes for the beams propagating CW and CCW in the cavity in opposite sense. Therefore, the ring laser operates at different resonance frequencies in the CW and CCW directions, and will provide a beat frequency at the output, proportional to the rotation rate as well as the area enclosed within the polygon.
A second type of optical gyroscope which relies on the Sagnac effect is the Fiber Optics Gyro (FOG), which employs a coil of optical fiber through which light waves are transmitted in opposite directions. When the FOG is at rest, the CW and CCW beams experience the same transit time through the coil, and therefore, when the two beams are coupled out of the coil and superimposed on a detector, they exhibit a nominally zero phase difference. When the FOG is rotated around its axis of symmetry, perpendicular to the plane of the optical coil, however, one beam of light travels with the rotation, and the other travels against the rotation, so that the two beams no longer have identical transit times, and will exhibit a phase difference at the output.
This phase difference is proportional to the rotation rate, as well as the area enclosed by the coil. Analysis of the interference pattern of the combined light waves at the photodetector, typically by means of a connected signal processor, provides an indication of rate and direction of rotation.
Winding lengths of low-loss optical fiber on a relatively small coil creates a large effective area, making it possible for a compact sensor to resolve very small rotation rates.
The manufacturing costs of these types of gyros, particularly the optical portion, are considerable, which can limit their application. To improve the gyros of the above types, so that less adjustment work is required and so that they can be mass-produced at low cost, part or all of the optical gyroscope can be located on a silicon integrated optics chip (IOC). In an Integrated Ring Laser Gyro (IRLG), the optical components of the ring resonator are replaced by an optical waveguide forming a closed loop on the integrated optics substrate. The other optical and electron-optical functions of the gyro can also be located on the same IOC. Realizations of this concept are known from, for example, U.S. Pat. No. 5,327,215 granted Jul. 5, 1994, and U.S. Pat. No. 5,408,492, granted Apr. 18, 1995.
An Integrated Fiber Optic Gyro (IFOG) is a variation of the FOG, which does not go as far with integration as the IRLG. In this case, the fiber optics coil is still present, but the optical and electro-optical functions of the IFOG are located on an Intergrated optics chip. Realizations of this concept are known from, for example, U.S. Pat. No. 5,194,917, granted Mar. 16, 1993, as well as U.S. Pat. No. 5,321,503, granted Jun. 14, 1994.
In the FOG discussed above, the output signal is manifested by a sinusoidally varying intensity pattern. The sensitivity of the device is governed by the gradient of the signal shape near the working point.
For maximum sensitivity, the working point of the sensor should be at the point of maximum gradient. This occurs when the two interfering light beams are a quarter wavelength out of phase, and is known as the quadrature point.
FOGs use a number of phase modulation methods to obtain quadrature. The most well known is to incorporate a piezoelectric device into the gyro loop operating at a frequency such that a nonreciprocal quadrature phase shift is introduced into the system. However, because of the reciprocity of the counterpropagating beams of light, it is not possible to effect the quadrature biasing by any manipulation of the length of the optical fiber sensor portion of the FOG.
It is clear that there is a need for a solid-state type of gyroscope to address the problems discussed above.